OK, do a search for a thread I started called A few Upgrades. Probably easier to search by name and then choose Threads started option. ( Link attached if I did it right, if not search time )
I listed all kinds of upgrades I did to my 95 Maristar. It will be very similar to your boat.
The short answer is Yes, you can add speakers to this boat. I added a couple of 6X9 higher up in the seating area. If I had to do it again, I would not have added them. I get plenty of sound from the lower mounted speakers. I'm not even talking about the Tower speakers.
Pics of the boat your looking at would help. $16K seems high without a system in place.